1. Field of the Invention
The present invention relates to a current resonance circuit and a power supply apparatus utilizing a current resonance circuit.
2. Description of the Related Art
FIG. 3 is a circuit diagram of essential portion illustrating an example of the power supply apparatus utilizing a current resonance circuit of the related art. First, the power supply apparatus utilizing a current resonance circuit of the related art will be explained with reference to FIG. 3.
This power supply apparatus 102 is a switching power supply apparatus comprising, as illustrated in FIG. 3, an oscillating circuit 104, first and second transistors 106, 108, a transformer 110, a capacitor 112, a rectifying circuit 114 and an oscillation control circuit 116. Here, the oscillating circuit 104, first and second transistors 106, 108, transformer 110 and capacitor 112 form a current resonance circuit 118.
The primary coil of transformer 110 and capacitor 112 are connected in series to form a serial circuit 120. The drain of the first transistor 106 is connected to the power supply line 122, the source is connected to one end of the serial circuit 120, while the other end of the serial circuit 120 is connected to the ground. Meanwhile, the drain and source of the second transistor 108 are respectively connected to both ends of the serial circuit120.
The oscillating circuit 104 oscillates in the frequency depending on a given control current and generates, as illustrated in the waveforms of FIG. 4, a couple of rectangular AC signals including phase difference of about 180 degrees, namely the ON/OFF control signals 124, 126 to respectively supply these signals to the gates 128 of first and second transistors 106, 108.
A rectifying circuit 114 rectifies the AC voltage induced on the secondary coil of the transformer 110 and then outputs a DC voltage as an output voltage of the power supply apparatus 102 through an output terminal 132. An oscillation control circuit 116 generates a control current based on the output voltage of rectifying circuit 114 and supplies it to the oscillating circuit 114 to control the oscillation frequency of the oscillating circuit 104 to keep constant an output voltage of the rectifying circuit 114. For example, when a load connected to the output terminal 132 is heavy, or a voltage supplied to the power supply line 122 is low and an output voltage of the rectifying circuit 114 is lowered, a control current output from the oscillating circuit 104 is increased, for example, so that the oscillating frequency of the oscillating circuit 104 becomes lower in order to maintain the output voltage of the rectifying circuit 114.
As explained above, the oscillating circuit 104 is controlled to lower the oscillating frequency when a load is heavy in the power supply apparatus 102 utilizing a current resonance circuit 118 of the related art, but if the oscillating frequency of the oscillating circuit 104 is excessively lowered, difference between the oscillating frequency and self-oscillating frequency of the current resonance circuit 118 becomes large loosing the resonance. As a result, the resonance condition is no longer maintained in the current resonance circuit 118 and the power supply apparatus 102 enters the control disabling condition.
For this purpose, the oscillating circuit 104 has a structure that the oscillating frequency is locked at the minimum frequency so that the frequency is no longer lowered without relation to a value of the control current from the oscillation control circuit 116 when the oscillating frequency is lowered up to the constant frequency. Thereby, the power supply apparatus 102 is prevented to enter the control disabling condition.
Characteristics of parts forming the power supply apparatus generally change depending on temperature and the parts include fluctuation. Accordingly, the oscillating frequency as a limit for leading the power supply apparatus 102 to the control disabling condition changes depending on temperature and is also different in each apparatus depending on fluctuation of parts. Therefore, on the occasion of setting the minimum frequency of the oscillating circuit 104 by adjusting, for example, a variable resistor or the like, such minimum frequency is set with allowance to a little higher value considering temperature characteristics and fluctuation of parts explained above.
However, as a result, variable range of oscillation frequency of the oscillating circuit 104 becomes narrow and therefore variable range of an input voltage (voltage of power supply line 122) and variable range of load become narrow.